Salinity – the awakening monster from the deep
Box 1 | Groundwater systems
Australian groundwater systems are not all alike. Recently they have been categorised into three main groups: local, intermediate and regional, each of which is further subdivided into four on the basis of their underlying geology.
Local groundwater systems
Local groundwater system catchments usually occur on a horizontal scale of about 1-3 kilometres and tend to occur in hilly terrain. Because of their relatively small size and the often low permeability of the underlying geology, these systems rapidly ‘fill up’ with water after land-clearing, and salinity and waterlogging may occur within a few decades. Such systems also respond quite rapidly to efforts to reduce recharge; well-designed and strategically located tree plantations, for example, could start to lower the watertable within a few years. Unfortunately, the worst salinity problems usually occur in intermediate and regional systems, where tree-planting is less likely to solve the problem.
Intermediate groundwater systems
Intermediate groundwater systems occur on a horizontal scale of 10-15 kilometres. These systems give rise to some of the more spectacular scenes of salinisation in Western Australia. Large-scale tree-planting – over much of the catchment – would usually be required in such systems to significantly reduce recharge.
Regional groundwater systems
Regional groundwater systems occur on a scale of 50 kilometres or more, usually in flat terrain. Limiting recharge is difficult because of the large surface area and also because regional groundwater systems often occur in semi-arid areas where tree-planting may not be economically viable. Moreover, efforts to lower watertables in intermediate and regional-scale groundwater systems is made more difficult by their generally low discharge capacities: in other words, since the terrain is often quite flat, the flow of groundwater to low points in the catchment and subsequent drainage out of the system can be very slow. So even if recharge is prevented, the watertable may take decades – and even hundreds of years – to return to previous levels.
Related sites
Salinity – groundwater flow
(National Land and Water Resources Audit, Environment Australia)
Australian groundwater flow systems contributing to dryland salinity
(National Land and Water Resources Audit, Environment Australia)
Posted August 2003.